Page:Popular Science Monthly Volume 23.djvu/811

Rh would surprise the system while it is in the process of accommodation. "The organization," says Dr. Pettenkofer, "is a prudent and faithful servant, which will deliver itself and its master from trouble if it is given time to set itself right and is protected against rude treatment." The body, even when exposed stark naked to the air, is not wholly without defense against heat and cold. It can, up to a certain point, itself regulate the expenditure of caloric by the intervention of the vaso-motor nerves that go to the capillaries of the skin. Cold provokes a shrinking of the little vessels, and, restraining the peripheric circulation, diminishes the radiation and the transpiration to such a degree as to protect the internal organs for a considerable time. Heat, on the other hand, dilates the vessels so that the blood flows to the surface and the caloric is in a certain way driven out. Unfortunately, this automatic regulator, the play of which is commanded by the nerves, is too easily disordered and its springs are too easily relaxed. We can doubtless fortify it by exercise, harden ourselves, and habituate the body to support inclement conditions; and there are peoples and persons who have done wonders in this direction; but the hardening process works under limitations, and its results are not within everybody's reach. The real regulators of the heat of the body are clothes.

The thinnest veil is a vestment in the sense that it moderates the loss of heat which radiation causes the naked body to experience. In the same way a cloudy sky protects the earth against too great cooling in spring nights. In covering ourselves with multiple envelopes of which we augment the protecting thickness according to the rigor of the seasons, we retard the radiation from the body by causing it to pass through a series of stages, or by providing relays. The linen, the ordinary dress, and the cloak constitute for us so many artificial epidermises. The heat that leaves the skin goes to warm these superposed envelopes; it passes through them the more slowly in proportion as they are poorer conductors; reaching the surface, it escapes, but without making us feel the chills which direct contact with the atmosphere occasions, for our clothes catch the cold for us. The hairs and the feathers of animals perform the same function as toward their skin, serving to remove the seat of calorific exchange away from the body. The protection we owe to our clothes is made more effectual by their always being wadded with a stratum of warm air. Each one of us thus has his own atmosphere, which goes with him everywhere, and is renewed without being cooled. The animal also finds under its fur an additional protection in the bed of air that fills the spaces between the hairs; and it is on account of the air they inclose that porous substances, furs, and feathers keep warm.

Experiments to determine the degree of facility with which different substances used for clothing allow heat to escape were made by Count Rumford, Senebier, Boeckmann, James Starck, and M. Coulier.